This invention deals with solutions for the problem of obtaining a friction force low enough to at least avoid jamming between a piston, specifically a piston comprising a container having an elastically deformable container wall, and the wall of an elongate chamber during the stroke, the chamber having different sizes of cross-sectional area's in its longitudinal direction, specifically those having different circumferential length's, when the piston is sealingly movable relative to said chamber.
A problem with embodiments of FIGS. 6, 8 and 9-12 (incl.) of WO 00/70227 may be that the piston may jam in the smaller cross-sections of the chamber having cross-sections with different circumferential sizes. Jamming may occur due to high frictional forces of the material of the wall of the pistons. These forces may mainly be created by the compression of the material(s) of the wall of the piston when the piston is moving from a first longitudinal position in the chamber having the biggest cross-sectional area to a second longitudinal position where the cross-sectional area and the circumferential size is smaller. FIGS. 1-3 (incl.) of the current patent application show examples of high frictional forces for non-moving pistons comprising a container in a non-moving chamber with or without internal pressure in the chamber. This results in high contact pressures between the piston and the wall of the chamber: jamming may occur.
A further problem may be that embodiments of pistons comprising a container of WO 00/70227 may leak their fluid, and thus may change their sealing capability. As in the solutions of the earlier mentioned problem for pistons comprising a container with an elastically deformable wall the sealing force is created by internal pressure, leakage may be an important problem.